My 04’ Mitsubishi Lancer runs hot at idle.?

I have a 2004 Mitsubishi Lancer(100k miles), and it runs hot at idle(sitting in one spot or sitting in park), but cools when driving.. when it runs hot, i let it sit for an hour or so and add coolant and it does fine for a couple of days. i changed the thermostat & it did okay for a few days then started doing... show more I have a 2004 Mitsubishi Lancer(100k miles), and it runs hot at idle(sitting in one spot or sitting in park), but cools when driving.. when it runs hot, i let it sit for an hour or so and add coolant and it does fine for a couple of days. i changed the thermostat & it did okay for a few days then started doing the same thing again so I added ‘bars leak liquid aluminum cooling system radiator and heater core stop leak’ & that stopped it for a few days. I also noticed when I take it on the highway it heats up just a little but once I come to a stop it almost instantly runs hot. Just recently was on the road and after about 45 minutes of driving, it started to heat up almost to red so I’m at a stop at the moment waiting on it to cool to add more coolant. I noticed a good bit of coolant leak(very first time it has had the leak) that looks like is coming from the reserve but the reserve is full. I’m trying to figure out where is the coolant going for me to have to keep adding nd this is the very first time it has leaked.. Any ideas of what this can be and what it would cost to get it fixed?
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